In recent years, the active matrix type liquid crystal display devices using as a switching element thereof a thin film transistor (hereinafter referred to as "TFT") formed by using an amorphous silicon (hereinafter referred to as "a-Si") film have been attracting attention. This is because there exists the possibility that a panel display having a large area, high accuracy, and high quality, namely a flat television, will be realized inexpensively by forming a TFT array with an a-Si film producible at low temperature on an inexpensive glass substrate.
In the construction of a display of a large area, however, the amount of resistance offered by the address wiring to be laid therein is increased because the gross length of the address wiring is inevitably increased to a marked extent. This increase in the amount of resistance of the address wiring entrains the problem of conspicuously delaying gate pulses to be given to switch elements and rendering control of a liquid crystal difficult. It is, therefore, necessary that the delay of gate pulses be avoided while retaining at least such parameters as wiring widths intact.
As one measure to avoid the delay of gate pulses, the idea of forming the address wirings with a wiring material which possesses as low resistivity as permissible may be conceived. At present, the Mo--Ta alloy film is often used as the material for the address wirings. Since this alloy film has such large resistivity as about 40 .OMEGA..multidot.cm, it is held that the resistivity of the Mo--Ta alloy film renders difficult the realization of a display of a large area. Particularly, the direct-view type display of high accuracy using about 1000 address wirings is considered to require a wiring material possessing resistivity of not more than about 20 .mu..OMEGA.cm.
The new wiring material of such quality as mentioned above is required to possess such characteristics as will be shown below in addition to the low resistivity mentioned above. Since the insulation between the wirings and the address wirings which are formed on an interlayer insulating film must be heightened by improving the step coverage of the interlayer insulating film formed on the address wirings, this new wiring material must be capable of being tapered.
The realization of a liquid crystal display device which, owing to the formation of address wirings with a wiring material of low resistance, is enabled to repress the delay of gate pulses and, at the same time, acquire necessary insulation and enjoy high reliability is yearned for. The desire of this kind has been expressed not merely for liquid crystal display devices having a display of a large area but equally for liquid crystal display devices having wirings and wiring intervals narrowed for the purpose of exalting the accuracy of display or liquid crystal display devices having an opening ratio improved by decreasing the width of wirings.
The conventional liquid crystal display devices have such problems as shown below in addition to those already remarked above. FIG. 5 is a cross section of a TFT (switching element) and a storage capacity part to be used in a liquid crystal display device.
By sputtering a Mo--Ta alloy, a gate electrode 2, address wirings, address wirings, and a Cs line 9 are simultaneously formed on a glass substrate 1 as shown in FIG. 5. Through the medium of a gate insulating film 3 which is formed thereon, an a-Si active layer 4 is superposed. On the opposite end parts of this active layer 4, n.sup.+ a-Si layers 5a and 5b are deposited. Then, through the medium of the gate insulating film 3, an ITO picture element electrode 8 is formed. Subsequently, an Al source electrode 6a having a part thereof connected to the n.sup.+ a-Si layer 5a, a drain electrode 6b having a part thereof connected to the n.sup.+ a-Si layer 5b and the picture element electrode 8, and data wirings are simultaneously formed.
The conventional TFT shown in FIG. 5 has a picture element electrode and data wirings formed in one and the same layer without intervention by an insulating film and, therefore, has the possibility of forming a short circuit and giving birth to point defects. To avoid the point defects, the configuration which has a source electrode, a drain electrode, and data wirings laid out first, an interlayer insulating film subsequently formed thereon, and a picture element electrode finally superposed thereon has been conceived and studied as a possible measure of improvement. To realize this configuration, the following requirements must be satisfied.
(1) The data wirings and others should possess excellent tolerance for the etchant for use on the interlayer insulating film and to the ITO etchant for the picture element electrode.
(2) For the purpose of improving the step coverage of the interlayer insulating film thereby heightening the insulation between the data wirings and the picture element electrode, the data wirings should be capable of being tapered.
Since none of the wiring materials developed to date fulfils the requirements mentioned above, it has been difficult to realize the configuration mentioned above and improve the reliability of a liquid crystal display device. Since it is important to lower the ratio of occurrence of point defects for the development of a display of a large area, the desirability of developing a liquid crystal display device of such high reliability has been finding growing recognition. For this reason, it is essential to develop a wiring material which satisfies the requirements mentioned above and also a target for the formation of a wiring.
Incidentally, the Al and the Ta type alloy form an oxide film on their surfaces and offer an increased contact resistance to the metallic wirings which are formed in the layer superposed thereon and, therefore, require a step for the removal of the oxide film from their surfaces. They further require a barrier metal to prevent ITO and Al from reacting with each other and, therefore, are at a disadvantage in inevitably increasing the number of steps of the process of manufacture.
An object of this invention is to provide a material for wirings having low resistance and permitting the work of tapering, a target for the formation of wiring, and a wiring thin film. A further object of this invention is to provide a material for wirings having low resistance and possessing high tolerance for the etchants for an interlayer insulating film, ITO, or the like, a target for the formation of wirings, and a wiring thin film. Another object of this invention is to provide a method for the production of a target for the formation of wirings, which method permits such a target for the formation of wirings as mentioned above to be manufactured with high repeatability.